(* An example of the caml language *)
(* Convert an integer to a string describing that integer *)

(* This exception is just in case something goes wrong. Don't worry though,
   nothing will go wrong. *)
exception Illegal_num


(* This is a trinary, three individual digits *)
type trinary = int * int * int


(* Convert a single ones digit to a string *)
let ones_to_string = function
  | 0 -> ""
  | 1 -> " one"
  | 2 -> " two"
  | 3 -> " three"
  | 4 -> " four"
  | 5 -> " five"
  | 6 -> " six"
  | 7 -> " seven"
  | 8 -> " eight"
  | 9 -> " nine"
  | _ -> raise Illegal_num


(* Convert a single tens digit to a string *)
let tens_to_string = function
  | 0 -> ""
  | 2 -> " twenty"
  | 3 -> " thirty"
  | 4 -> " fourty"
  | 5 -> " fifty"
  | 6 -> " sixty"
  | 7 -> " seventy"
  | 8 -> " eighty"
  | 9 -> " ninety"
  | _ -> raise Illegal_num


(* Convert teens to string *)
let teens_to_string = function
  | 0 -> " ten"
  | 1 -> " eleven"
  | 2 -> " twelve"
  | 3 -> " thirteen"
  | 4 -> " fourteen"
  | 5 -> " fifteen"
  | 6 -> " sixteen"
  | 7 -> " seventeen"
  | 8 -> " eighteen"
  | 9 -> " nineteen"
  | _ -> raise Illegal_num


(* Convert three digits to hundreds, tens, ones string *)
let trinary_to_string (a,b,c) =
    (if a > 0 then (ones_to_string a) ^ " hundred" else "")
  ^ (if b = 1 then (teens_to_string c)
      else (tens_to_string b) ^ (ones_to_string c))


(* Remove three digits from the right side, return them and the new number *)
let remove_triad n =
  let a = (n mod 1000) / 100 in
  let b = (n mod 100) / 10 in
  let c = n mod 10 in
  ((n / 1000),(a,b,c))


(* Recursively build the string *)
let rec iter_triads n postfix =
  if n > 0 then
    let (new_num,tri) = remove_triad n in
    (iter_triads new_num (List.tl postfix))
      ^ (trinary_to_string tri)
      ^ (List.hd postfix)
  else ""


(* Main function, given an int convert to a string *)
let int_to_string n =
  let postfixes = [""; " thousand"; " million"; " billion"] in
  if n = 0 then "zero" else 
    let str = iter_triads n postfixes in
    if (String.length str) > 1 then
      (String.sub str 1 ((String.length str) - 1))
    else ""


(* Program entry point. Test on a list of integers *)
let _ =
  let nums = [0;1;13;123;5434;1234567891234;57;1066123001] in
  let strs = List.map int_to_string nums in
  List.iter (fun s -> print_string s ; print_newline()) strs